(copyright) 2001 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR xc2xa71.71(d).
This invention relates to harmonically-converted solid-state lasers and, in particular, to quasi-CW diode-pumped UV laser systems and processing methods employing them, such as for forming vias in circuit boards.
Different types of lasers systems have been employed to drill vias at point-to-point target areas on electronic devices or work pieces such as printed circuit boards (PCBs). The following discussion is presented herein only by way of example to diode-pumped, solid-state ultraviolet (UV) laser systems and work piece targets and should not be considered limiting to the scope of invention.
When an acousto-optically (A-O) Q-switched, continuous-wave (CW) diode-pumped, solid-state (DPSS) laser system, such as Electro Scientific Industries, Inc.""s (ESI) Model 5200 which includes a Light Wave Electronics"" (LWE) Model 210 laser, is employed to create vias, the pumping diode or diodes remain active continuously. Laser emission is prevented by closing the Q-switch whenever the positioning system is moving to a new target area on the work piece. After the positioning system is aligned to the new target area, the laser system delivers a laser output containing one or more laser pulses by opening the Q-switch at a predetermined repetition rate.
The LWE Model 210 employs two 20 Watt (W) CW diodes for pumping and generates 3 W of UV output power at 10 kHz repetition rate. The CW pumping current to the diodes is limited by thermal loading of the diodes. If an application warrants greater UV output power, then either more diodes or diodes with higher current/power must be employed, such as two 30 W diode laser bars or four 20 W diode laser bars. About 8 W of UV output power can be expected from such designs. However, the diode laser bars, themselves, constitute a major portion of the cost of a DPSS laser, and using more diodes or diodes with higher pumping output power to boost laser output power will significantly increase the cost of the DPSS laser.
Other pumping schemes are, however, available for a laser design, such as pulse pumping and quasi-CW pumping. An electro-optically (E-O) Q-switched pulsed DPSS UV laser, such as early versions of Lambda Physics"" UV xe2x80x9cGatorxe2x80x9d Model, emits one laser pulse for each pumping pulse, providing a higher laser pulse power but at low pulse repetition rates. The pumping duration time is limited to a few hundred microseconds (xcexcs) so the laser output pulse repetition rate is typically limited to below 2 kHz. This pumping scheme is not preferred for drilling vias, because this scheme adversely affects drilling throughput.
Traditional quasi-CW pumping resembles pulse pumping but exhibits longer pumping duration time at a lower peak pumping power. The pumping scheme can exhibit a pumping repetition rate of about 1 to 2 kHz, and the pumping duration time can be from a few hundred microseconds to a few milliseconds (ms), based on the repetition rate and the duty cycle of the diodes used. This pumping scheme allows pumping to a higher level than, does CW pumping because the diode xe2x80x9crestsxe2x80x9d (and thermal loading reduces or stops) whenever the pumping is off. Therefore, the laser output power can be higher during the pumping time period compared to that of a comparable CW-pumped laser. The laser output is controlled by regulating the current to the diode(s). The pumping repetition rate of this pumping scheme is, however, still a serious drawback. Typical applications for quasi-CW pumping include those that utilize a long laser pulse width and a modest peak power, such as laser bonding and welding.
A laser system that includes a pumping scheme that facilitates a higher usable laser power and a higher repetition rate to increase drilling throughput is therefore desirable.
Conventional UV laser via drilling systems employ a standard frequency conversion scheme to convert the laser""s fundamental wavelength in the infrared (IR) region to the UV. Such systems preferably employ high UV power and a high pulse repetition rate to achieve high throughput via formation, hence A-O Q-switched DPSS laser systems have heretofore been preferred for drilling vias.
A commercially desirable system would prefer higher UV power for reducing the via drill time, or to make acceptable vias on some xe2x80x9chard to drillxe2x80x9d materials, such as copper and FR4. Thus, a high UV output power (5 to 15 W or greater) at a high pulse repetition rate (a few kHz to a few tens kHz or greater) would be preferred.
Also, to be commercially useful, via formation on PCBs, for example, demands a laser system to be capable of making 300 to 400 vias per second. Thus, the laser positioning system has to move to 300 to 400 new locations every second. Typically, it takes the laser system less than one ms to drill one via, but in some cases longer than one ms to move to a new location for a next via. Hence, the time for the laser being ON is actually less than the time the laser is OFF, which makes the use of conventional laser pumping schemes quite inefficient.
The present invention provides a quasi-CW diode-pumped, A-O Q-switched solid-state UV laser that synchronizes the timing of the quasi-CW pumping to avoid or reduce pumping while the positioning system is moving from one target area to the next target area and to increase the pumping level to a higher xe2x80x9csuper pumpingxe2x80x9d level beyond the CW-pumped level while drilling vias. Thus, the available UV power for via formation is higher even though the average pumping power capability, and thermal loading of the pumping diodes, remains the same as for conventional CW pumping with conventionally available laser diodes. The higher available UV output power enables higher repetition rates to be employed. The quasi-CW pumping current profile can be further modified to compensate for UV power drifting due to the nature of harmonic generation or caused by local heating of the nonlinear crystal in order to realize a preferred UV pulse amplitude profile. The current supplied to the laser pumping diode or diodes during low pumping intervals can be adjusted to optimize the stability and/or lifetime of the laser pumping diode(s). Fewer and less expensive diodes can be employed to produce greater output power, and the diodes can be replaced less frequently, greatly reducing the cost of the laser system and its maintenance.
Such a quasi-CW diode-pumped, A-O Q-switched, solid-state UV laser is new; the synchronization of the quasi-CW pumping with the beam scanning is new; and the use of such a laser system for via formation is new.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.